In wavelength conversion technique based on nonlinear optical phenomena a condition known as phase matching is imposed on wave vectors, which corresponds to the law of conservation of momentum between propagating light waves and played a very important role in the field. A variety of phase matching conditions have been proposed, including, for example, a forward/backward phase matching, collinear/non-collinear phase matching, and pseudo-phase matching (QPM), and as a result, wavelength conversion efficiency has been improved and an effective wavelength range have been increased. Among others, optical parametric oscillation (OPO) utilizing backward phase matching presented by Harris in 1966 has been known as nonlinear optical wavelength conversion with unique characteristics (Non-Patent Document 1). This phase matching increases quantum conversion efficiency to the maximum, while adopting an extremely simple optical system (Non-Patent Documents 2 to 4). This is brought by a feedback effect occurring between the light waves propagating in the opposite direction in the nonlinear optical crystal.
An electromagnetic wave in a frequency range of the order of 0.1 THz to 100 THz (3 μm-3 mm in wavelength) is sometimes referred to as a terahertz wave and a wide application is expected. Therefore, improvement in practical use has been sought extensively, in a method of generating high output and high efficiency, a frequency variable light source over a wide range, room temperature operability, and so on. In particular, a coherent terahertz wave is expected to exhibit strong interaction with a substance and is considered as promising as the wavelength region includes a so-called fingerprint region; therefore, its efficient generation method has been searched in application fields such as detection of trace substances. The present coherent terahertz wave, in most cases, obtained by nonlinear wavelength conversion using a light source of a near-infrared laser. In order to realize the nonlinear wavelength conversion, a nonlinear optical crystal is disposed in an external resonator having mirrors or the like, which is provided with pump light for excitation. Most of such devices require laser light sources for a plurality of wavelengths and are large-scaled. The nonlinear wavelength conversion is also effective in a terahertz wave region, and terahertz wave generation has been realized by applying such techniques as in induced polariton scattering, difference frequency generation, light rectification effect, and so on.